In many antitumor agents, their antitumor activities are shown only after the agent is absorbed in tumor cells. Therefore, it has been thought that drugs having low molecular weights and stable structures are effective as antitumor agents in order to make it easy for the drugs to be taken in through receptors of tumor cells.
However, the antitumor agent that can be easily absorbed in tumor cells through the receptors is also easily excreted from the cells conversely. Accordingly, there has been a problem that the antitumor agent cannot stay in tumor cells for a sufficient length of time to show its antitumor activity. In addition, such an antitumor agent that is easily absorbed through the receptors is easily taken in normal cells as well as in tumor cells and, therefore, is difficult to be used for topical treatment and causes a problem of the side effect due to injury of the normal cells.
On the other hand, high molecular compounds, such as a catenane compound in which two ring-shaped molecules are bound to each other like a chain with no covalent bond, and a rotaxane compound having a structure in which a linear molecule stays through a ring-shaped molecule with no covalent bond, have large molecular weights and have high degrees of freedom in positional relationship between two structural units. Therefore, it is thought that the high molecular compounds that have been once taken in cells are not easily excreted. Accordingly, these high molecular compounds are promising candidates for antitumor agents. Antitumor agents utilizing these high molecular compounds are disclosed in Patent Document 1 (catenane compound) and in Patent Document 2 (rotaxane compound) as below.